Wheezing illnesses with rhinoviruses (HRV) are important causes of wheezing in all age groups, and infants who wheeze with HRV appear to be high risk for developing asthma. Unfortunately, there is little understanding of viral factors that promote lower respiratory symptoms and wheezing. Our overall hypothesis is there is a subset of virulent HRV serotypes/strains which are more likely to cause significant infections of the lower airway and cause acute exacerbations of asthma. We further hypothesize that the degree of virulence is dependent on strain-specific differences, likely to reside in 3C and 2A viral protease sequences. For most viruses, strainspecific differences in pathogenicity have been identified, but this information is lacking for HRV because of the large number of serotypes and limitations of conventional serotyping techniques. Together with collaborators at the University of California-San Francisco, we have developed new molecular technologies to detect, classify, and characterize the genetic structure of HRV. Our preliminary data demonstrate the utility of these assays, and these methods have in fact led to the discovery of many unique strains of HRV. We propose to characterize the strain and genetic structure of HRV that caused wheezing in COAST participants during infancy, and to collect new specimens from 8-10 year-old children with asthma to identify more virulent strains associated with acute exacerbations. Mechanisms of strain-specific virulence will then be evaluated ex vivo using cultured epithelial cells, monocytes, and molecular analysis of viral proteases. Collectively, these experiments will provide information that can be used to develop new strategies for prevention and/or treatment of virus-induced wheezing illnesses in children.